1. Field of the Invention
Building air conditioning systems are currently in use today which are of the variable air volume type; that is, in order to control the temperature within a conditioned zone, the volume of conditioned air, either heated or cooled, supplied to the zone is varied in accordance with the heating or cooling load within the zone. Conventionally, air valve means are provided for throttling the flow of conditioned air, which air valve means may be operated by an electric motor, typically of the low voltage type providing bi-directional operation.
In order to sense the heating or cooling load within the conditioned zone, it is common practice to provide a zone thermostat, either located directly within the conditioned zone or in a return air passage at a position so as to sense the temperature of return air from the zone. Associated control circuitry has been provided in order to operate the air valve means in response to the heating or cooling load within the zone, as sensed by the thermostat. In order to provide proper operation of the system during both the heating and cooling modes, it has been necessary to provide control circuitry having suitable changeover capability, it being further desirable to limit both the minimum and maximum flow of conditioned air to the zone in order to maintain proper ventilation requirements and prevent drafts or excessive noise within the conditioned zone.
2. Description of the Prior Art
Prior art temperature controls of which applicants are aware, and which are exemplified by U.S. Pat. Nos. 3,690,548; 3,682,381; and 3,734,192; include a bi-directional electric motor for opening and closing suitable air valve means in order to vary the flow of conditioned air to a conditioned zone. Changeover between heating and cooling is provided in these systems through the use of a duct thermostat having switch means which are disposed directly in series with the electrical conductors which energize the aforesaid bi-directional motor. Upon a change in the duct temperature, the heating and cooling signals from the zone thermostat are simply applied to opposite windings of the bi-directional motor in order to reverse its sense of operation with respect to the thermostatic signal. The '548 patent further includes provision for limiting the maximum flow through the air valve by providing a switch which, upon occurrence of a flow exceeding the maximum desired, energizes the closing windings of the valve motor.